The following definition comes from the American Academy of Periodontology "Parameter on Early-Onset Periodontitis".
Early-onset periodontitis encompasses distinct types of periodtitis that affect young persons who, in most cases, otherwise appear healthy. Early-onset periodontitis may be distinguished from adult periodontitis by the age of the disease onset, the rapid rate of disease progression, mainfestations of defects in host response, and composition of the associated subgingival microbial flora. Early-onset pereodontitis includes threes forms: Prepubertal periodontits, localized and generalized juvenile periodontitis, and rapidly progressive periodontitis.